A contact is a semiconductor chip-to-package interconnect technology. The advantages in the contacts lie in the extendibility to finer pitch and the superior electromigration performance. The contacts may be made from copper and the finer pitch is due to the contact's vertical sidewall.
In traditional copper contact technology, the contacts are formed upon a semiconductor wafer via photoresist defined plating. Typically, a dual layer of sputtered metals is formed upon the semiconductor, a photoresist layer is formed upon the dual layer, and the photoresist layer is patterned. The contact is electroplated within the patterned photoresist. Subsequent to plating, the photoresist is stripped from the semiconductor wafer utilizing a photoresist stripping solution.
The bottom metal layer is typically used as both a barrier and an adhesion layer to the underlying wafer material(s). The second layer is a current carrying or seed layer which is typically some form of copper or copper alloy utilized in the contact electroplating fabrication.
These dual metal layers are particularly beneficial when lead plating due to the high activation energy needed to be overcome for lead ions to deposit as lead in the absence of tin. The Restriction of Hazardous Substances Directive (RoHS) restricts the use of certain hazardous substances in electrical and electronic equipment and has driven the electronics industry to move away from solders that contain lead. Therefore, as tin based plating has become prevalent, it has been found that tin is more noble in solutions than copper and has a low activation energy for tin ions to convert to tin (i.e., Sn2++2e−→Sn). As a result, tin can deposit onto the exposed seed layer to which the photoresist has been applied.
When tin is present on the surface of the seed layer, it lowers the activation barrier needed for lead ions to deposit onto a surface. As a result, the seed layer becomes covered with metal species (e.g., Cu3Sn, SnO, SnO2, Pb, etc.) that inhibit or may not be removed with photoresist stripping solutions. One resolution to prevent the metal specie deposition upon the seed layer is to segregate lead wafers (e.g., semiconductor wafers utilizing lead) from lead free wafers for stripping and to utilize a nitrogen or argon ash to remove the deposited metal specie from the seed layer prior to etching. However, this resolution leads to the metal species depositing onto the walls of the ash chamber that require increased costly chamber wall kit changes.